1. Field of the Invention
The present invention relates to a hydraulic valve assembly for supplying a pressurized working fluid from a hydraulic pump to an actuator, such as a hydraulic cylinder, a hydraulic motor and the like.
2. Description of the Related Art
FIG. 1 shows a typical construction of the conventional hydraulic valve assembly. The conventional hydraulic valve assembly has a valve body 1, in which a spool bore 2 is defined. The spool bore 2 is communicated with first and second inlet ports 3 and 4, first and second actuator ports 5 and 6, first and second tank ports 7 and 8. A valve spool 9 is slidably disposed within the spool bore 2 for selectively establishing and blocking fluid communication through respective ports. The valve body 1 is further formed with a pump port 10 which is selectively communicated with and blocked from the first and second inlet ports 3 and 4 by a load sensing valve 11. A first suction safety valve 12 is disposed between the first actuator port 5 and the first tank port 8. Also, a second suction safety valve 13 is provided between the second actuator port 6 and the second tank port 8. It should be noted that the first and second actuator ports 5 and 6 are connected to an actuator 17, the first and second tank ports 7 and 8 are connected to a tank 18, and the pump port 10 is connected to a pump 19.
In the hydraulic valve assembly constructed as set forth above, the valve spool 9 is formed with circumferential grooves 14 and 15 and lands 16 for selectively establishing and blocking fluid communication between respective ports. With the illustrated construction, sealing dimensions between the first actuator port 5 and the first tank port 7 and between the second actuator port 6 and the second tank port 8 are small to possibly cause leakage of the fluid from the first and second actuator ports 5 and 6 to the first and second tank ports 7 and 8.
Namely, in the construction set forth above, seal between the first actuator port 5 and the first tank port 7 and the second actuator port 6 and the second tank port 8 is established by sealing engagement between the inner periphery of the spool bore 2 and the lands 16 of the valve spool 9. Due to restriction of the overall size of the valve body 1 and of the stroke of the valve spool, the axial dimensions L1 of the lands 16 between the first and second actuator ports 5 and 6 and the first and second tank ports 7 and 8 become too small to satisfactorily establish the seal.
Also, in the prior art, the spool bore 2 and a plurality ports must be formed in the valve body 1 with high precision, machining is very difficult and thus cost intensive.
Furthermore, when a plurality of hydraulic valves assemblies are coupled to form a stack-type hydraulic valve assembly, the valve bodies are stacked and secured by means of stack bolts. In such case, the tightening torque of the stack bolts may cause deformation of the valve body to block sliding motion of the valve spool. To avoid this, in the prior art, the finishing of the spool bores has to be performed after assembling the valve bodies. Therefore, once the spool bore is finished, additional hydraulic valve assembly cannot be coupled.